1. Field of the Invention
The present invention relates to a high speed thermal printing system, and specifically a system capable of printing graphic material (for example, well logs) at relatively high speeds under adverse environment conditions while maintaining constant image quality.
2. Background
Basic high-speed thermal printers comprise a fixed thermal printhead, a resilient drive roller which is driven by a stepping motor, an electronic control circuit, and a power supply. The thermal printhead usually has a linear array of resistance heater elements, typically 200 elements/inch, across the width of the printhead. The printhead also contains a shift register stage, a latch stage, and a current driver for each heater element. The printer operates by pressing thermally sensitive paper (thermal paper) against the thermal printhead through the use of a resilient drive roller. The data to be recorded are shifted serially into the shift registers transferred to the latches, and then finally recorded by activating the current drivers with a print signal, or strobe pulse. New data can be shifted into the shift registers while the previous line of data stored in the latches is being printed.
Typical control circuits for thermal printheads account for the fact that the thermal elements in the printhead which were activated on a previous image line have a residual temperature which placed those elements at a higher temperature in relation to those elements which were not used in printing the previous image line. The usual solution which has been employed for that problem is to use a full length strobe pulse for heater elements which were not active during the printing of the previous image line, and a shorter strobe pulse for those heater elements which are used to print both the present and the previous line. This solution, however, requires shifting data into the thermal printhead twice for each image line being printed. The technique also permits intentional variation of density for gray scale presentations. However, in that case data must be shifted into the thermal printhead once for each shade of gray. The result is that the printing speed is limited.
Additionally, present thermal printers, as described above, are not able to produce uniform image quality when printing speed and ambient temperature conditions vary. This is because once a printhead element is heated to a certain temperature, it does not cool immediately upon removal of the driving current. Variations in either the printing speed or ambient temperature will affect the rate at which the printhead element cools. These two variables will thus cause the image density to vary unless they are compensated for.
Most present thermal printers are also not able to handle the high current axed power levels required to print an "all black" image. The useful life of the printhead is shortened considerably if such "all black" imaging is done repetitively. Furthermore, the power needed for present thermal printers to generate "all black" images imposes an increased power supply requirement with attendant increases in size and cost. This same factor also limits the ambient temperature conditions under which present thermal printers can operate. In order to operate under conditions of high ambient temperature, thermal paper must be used which is relatively insensitive so that it can be stored without darkening due simply to high ambient temperature. Currently available thermal printers have problems with such insensitive thermal paper for the same reasons that they have problems generating "all black images," i.e., shorter printhead life and increased power supply costs.
In summary, currently available thermal printers are not able to print 16-level gray scale images at speeds on the order of 1-inch per second due to the time required to shift data into the thermal printhead 16 times per image line. Also, available thermal printers which attempt to print large-area "all black" images at high speed cause overheating of the thermal printhead, and require larger and more costly power supplies. For the same reason, these printers have problems when relatively insensitive thermal paper is used.
The present invention was developed to overcome the problems discussed above. It is an object of the present invention to provide an electronic thermal printing system capable of printing in either a gray scale or graphics mode at high speeds. A further object of the present invention is to minimize the power requirements for generating a given image, thus facilitating the use of relatively insensitive thermal paper. It is still a further object of the present invention to provide images of uniform quality in spite of variations in printing speed, ambient temperature, and line to line variation in the use of particular resistance elements.